1. Field of the Invention
The present invention relates to method and apparatus for removing contaminants, such as nitrogen oxides, sulfur oxides, particulates, heavy metals and other acid gases, from gas streams arising from chemical, metallurgical, partial and full combustion processes, as for example, exhaust streams generated exhausts from engines or boilers on mobile sources such as sea going, naval or other vessels.
2. Description of the Prior Art
The use of ozone for oxidizing nitrogen oxides is described in U.S. Pat. Nos. 5,206,002; 6,162,409; 6,649,132; and 7,303,735. These chemistries and techniques are directed towards high levels of nitrogen oxides removal (around 90%) and require 1.5 moles of ozone per mole of nitrogen oxide present in the gas stream. Configuring these processes to operate at lower levels of nitrogen oxides removal causes both economic and process challenges.
Combustion and chemical processes generally result in gas streams containing contaminants that need cleanup before being exhausted to the atmosphere. Many industrial processes, power generating utilities, combustion sources, stationary and mobile sources such as engines, boilers, kilns and the like use solid fuels or low cost hydrocarbon fuels that contain sulfur, chlorine, nitrogen and metal compounds in hydrocarbons which result in exhaust gases that contain contaminants such as acid gases, particulate matter and heavy metals. To comply with stricter environmental rules mandated by legislation and a greater concern for the environment, combinations of scrubbing (wet or dry) and particulate capture devices such as electrostatic precipitators (ESP), wet ESP and bag house are increasingly preferred for emissions control of acid gas and particulate matters.
Nitrogen oxides found in most combustion exhaust streams are in the form of nitric oxide (NO), which is mostly insoluble in water and not very reactive. Nitric oxide is not removed by most wet or dry scrubber capture devices. Therefore, to control nitrogen oxides emissions, the two major options are to lower nitrogen oxides formation at the source by modifying combustion or secondly treating nitrogen oxides in the exhaust gas stream using post combustion techniques.
Primary techniques used for reducing nitrogen oxides formation by modifying combustion are low nitrogen oxides burner (LNB), flue gas recirculation (FGR), staged combustion and over fire air (OFA). In most applications these technologies are not adequate for removing nitrogen oxides from combustion gas streams and post combustion techniques, such as selective catalytic reduction (SCR) and selective non-catalytic reduction (SNCR), become necessary to achieve the desired nitrogen oxides reduction.
Both SCR and SNCR processes realize good results but also have limitations. Ozone based oxidation technologies have recently gained success as an alternative post combustion technique, especially when an application is not suitable for SCR. Ozone based processes as described in U.S. Pat. Nos. 6,162,409, 5,206,002 and 7,303,735 provide multi-pollutant removal approaches and they have been implemented on flue gases that arise from gas and coal fire boilers removing multiple pollutants including nitrogen oxide, sulfur oxides, particulates, etc. Ozone based processes are also industrially practiced in lowering emissions in other applications such as metal pickling processes, fluidized catalytic cracker (FCC) regenerators, metal recovery furnaces and sulfuric acid manufacture.
Coal fired boilers with low nitrogen oxides burners and staged combustion often attain nitrogen oxides in 0.25 to 0.4 lb/MMBTU cost effectively whereas regulations require nitrogen oxides emissions in a range of 0.1 to 0.15 lb/MMGBTU i.e., post combustion technology that can cost effectively offer 40 to 70% reduction.
The methods disclosed in U.S. Pat. Nos. 6,162,409, 5,206,002, 6,649,132 and 7,303,735 use chemistry of nitrogen oxides reaction with ozone by forming higher oxides of nitrogen, especially the pentavalent form or higher which are quite water soluble and readily removed by wet scrubbing. The stoichiometric amount of ozone required to convert one mole of NOX, in the form of NO, to the pentavalent form is about 1.5 moles of ozone and 0.5 moles if NOX is in the form of NO2.
Although these ozone based methods for removing nitrogen oxides from combustion streams are effective at achieving ultra low levels of nitrogen oxides emissions in the treated gas stream, there is need for an improved process for partial removal of nitrogen oxides that addresses the economic, regulatory and process challenges presented by emission controls.